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Part III: Near-Earth Objects

ABSTRACT: The distribution of known asteroids in Earth-approaching orbits is a biased sample, but does lead to predictions where objects may be found. To date, searches for near-Earth asteroids have concentrated on the opposition direction, but it is obvious that as systems push to fainter limiting magnitudes more asteroids will be discovered closer to the Sun. These will include Earth-approaching asteroids observed away from the Earth and objects interior to the orbit of the Earth that will never pass through opposition. Least-squares fits to the density distribution of known near-Earth asteroids are presented as preliminary functions to be debiased in order to obtain a true distribution. Some of the biases are reviewed. It appears, especially if searches are made closer to the Sun, that with some reasonable assumptions, the biases that exist may be described, leading to a better understanding of the dynamics that result in the observed distribution of Earth-approaching asteroids. CCDs promise to supplant photography as the preferred search method.

ABSTRACT: Near-Earth asteroids have relatively short (10-100 Myr) lifetimes, and the population must be replenished rapidly compared to the age of the solar system. We discuss the sources and mechanisms of replenishment, as evidenced by observations of asteroids, meteorites and meteors, and by dynamical theories. We also discuss the uncertainties in observations and theories, and the extent to which they influence our understanding of the composition and distribution of near-Earth asteroids.

ABSTRACT: Asteroids are a potentially rich source of a variety of materials in the solar system. Some of them are also relatively easy to access because they have been perturbed into near-Earth orbits. Most of our information on asteroid surface compositions comes from telescopic reflectance spectroscopy, and most of our information on asteroid bulk compositions has been determined from the study of meteorites. In this chapter, we summarize the current state of knowledge of asteroid compositions, using the information from both these sources. We conclude with a discussion of some of the remaining controversies.

ABSTRACT: The meteorites that fall on Earth are necessarily samples of the near-Earth population of small solar system bodies, and hence are a very useful guide to the nature of the materials available in near-Earth space. We review the compositions of the most abundant classes of noncarbonaceous meteorites that fall on Earth for the purpose of identifying economically attractive uses of near-Earth asteroid materials.

ABSTRACT: Samples from the Moon and most near-Earth asteroids are depleted in the volatile elements carbon and hydrogen. Only one class of asteroids is rich in these elements: carbonaceous asteroids. No carbonaceous asteroid has yet been sampled directly. Our knowledge of their composition is based on the study of carbonaceous meteorites. A wide variety of evidence suggests that near-Earth carbonaceous asteroids are the source of most carbonaceous meteorites. As about one third of the classified near-Earth asteroids appear to be carbonaceous, we can expect to find an abundance of volatile resources among the near-Earth asteroids. For space missions in the past, it has been necessary to bring all supplies from Earth. For the manned lunar base and Mars exploration missions of the future, we have the option of "living off the land." For example, space flight would be much less expensive if propellant were available in orbital fuel depots. Spacecraft could then be launched with empty tanks, dramatically reducing the payload mass and launch cost. Carbonaceous ore could be processed into propellant identical to that used by the space shuttle's main engines. When heated, the ore releases large quantities of water, carbon monoxide, and carbon dioxide. A fuel production plant could extract and process the water to provide a steady supply of hydrogen and oxygen propellant. Once in operation, the fuel plant becomes part of the growing infrastructure of the space economy. If we invest wisely in this infrastructure, it will yield tremendous economic benefits for decades to come. This chapter begins by discussing the economics of asteroid mining. Some simple volatile products are then identified. Production processes are suggested, with an emphasis on existing industrial practice. Finally, the mining environment is detailed, using the moons of Mars as well-studied examples.

ABSTRACT: The spacecraft flybys of comet Halley in 1986 confirmed Whipple's icy conglomerate model for cometary nuclei and showed that comets are far richer in volatiles than any other class of solar system bodies. Water is the most abundant volatile, comprising roughly 80% of the gas flowing out from the nucleus. CO is next with a content of 15% relative to water, though with at least half of that coming from an extended source in the cometary coma, most likely hydrocarbon dust grains. The detection of large numbers of hydrocarbon grains was one of the more significant discoveries of the Halley flybys, as was the groundbased observation that CN occurs in jets, again suggesting an extended source. Estimates of the total dust-to-gas ratio for Halley range as high as 2:1, indicating that a substantial fraction of the volatiles may be tied up in solid hydrocarbons rather than ices. The role of clathrates (if any) in trapping more volatile ices is not yet understood. If Halley can be taken to be representative of all short-period comets, which to first order we think it is, then the short-period comets provide a significant source of volatiles in near-Earth space. This resource is more difficult to reach dynamically than the near-Earth asteroids, but the high volatile content may justify the additional effort necessary. In addition, there is considerable evidence that at least some fraction of the near-Earth asteroids are extinct cometary nuclei which have evolved into asteroidal orbits, and which may contain significant volatiles buried beneath an insulating lag deposit crust of nonvolatiles. Our knowledge of comets will be greatly enhanced in the near future by the Comet Rendezvous Asteroid Flyby mission now under development by NASA, and by the proposed Rosetta mission, a joint effort by ESA and NASA to return a cometary sample to Earth for detailed analysis.

ABSTRACT: There exist frequent, low delta-V mission opportunities to many bodies in the complete near-Earth asteroid (NEA) population: about 250 of these asteroids larger than 1 km diameter are easier to reach than the Moon's surface. Opportunities for fast missions of less than one year duration that require less delta-V than a round-trip lunar mission occur about 9 to 10 times each year on average to a near-Earth asteroid larger than 0.5 km in diameter. However, less than 10% of the approximately 1,700 NEAs larger than 1 km have been discovered through mid-1991. Hence, a significant increase in the discovery rate of NEAs is needed to realize the mission potential in the NEA population. This point is illustrated by the fact that the best candidate (by far) for a fast-trip mission is the recently discovered asteroid 1991JW which was unknown at the time of the Tucson Conference on "Resources of Near-Earth Space," in January 1991. Given that there are many opportunities to visit these asteroids and that a large investment will be made in a Mars landing mission as part of the Space Exploration Initiative (SEI), a program of robotic and astronautic missions to NEAs would provide significant additional return on that investment by: (1) providing a precursor to a Mars mission with flight testing of hardware and mission operations (other than those needed for Mars surface operations); (2) providing a highly visible program milestone, sustaining momentum for the SEI in the interval between lunar base establishment and the Mars landing mission; (3) providing a large science return through in situ observations and returned samples; and (4) carrying out an assessment of the resources available in the NEA population that could be used for any future large-scale space development.